As described in the above-referenced '287 Halford et al application, in a TDMA cellular communication system, a simplified illustration of which is diagrammatically shown in FIG. 1, communications between a base station BS and a desired user 11-1 in a centroid cell 11 are subject to potential interference by co-channel transmissions from users in cells dispersed relative to the cell of interest (cell 11), particularly immediately adjacent cells shown at 21-71. This potential for co-channel interference is due to the fact that the same frequency is assigned to multiple system users, who transmit during respectively different time slots.
In the non-limiting simplified example of FIG. 1, where each cell has a time division reuse allocation of three (a given channel is subdivided into three user time slots), preventing interference with communications between user 11-1 and its base station BS from each co-channel user in the surrounding cells 21-71 would appear to be an ominous task--ostensibly requiring the placement of eighteen nulls in the directivity pattern of the antenna employed by the centroid cell's base station BS.
In accordance with the invention disclosed in the '287 application, this problem is successfully addressed by determining the times of occurrence of synchronization patterns of monitored co-channel transmissions from users in the adjacent cells, and using this timing information to periodically update a set of amplitude and phase weights (weighting coefficients) for controlling the directivity pattern of a phased array antenna. Namely, the weighting coefficients are updated as participants in the pool of interferers change (in a time division multiplexed manner), so as to maintain the desired user effectively free from co-channel interference sourced from any of the adjacent cells.
In addition to being applied to the weighting elements, the updated weighting coefficients are stored in memory until the next cyclically repeating occurrence of the time slot of the last (in time) entry in the current pool of co-channel participants. In response to this next occurrence, the set of weight control values for the current pool is updated and used to adjust the phased array's directivity pattern, so that the nulls in the directivity pattern effectively follow co-channel users of adjacent cells. The newly updated weight set is then stored until the next (periodically repeated) update interval for the current co-channel user pool, and so on.
Since the maximum number of nulls than can be placed in the directivity pattern of a phased array antenna is only one less than the number of elements of the array, the fact that the number of TDMA co-channel interferers who may be transmitting at any given instant is a small fraction of the total number of potential co-channel interferers (e.g., six versus eighteen in the above example) allows the hardware complexity and cost of the base station's phased array antenna to be considerably reduced. However, because the locations of co-channel interferers and therefore the placement of nulls is dynamic and spatially variable, the antenna directivity pattern must be controlled very accurately; in particular, excessive sidelobes that are created by grating effects customarily inherent in a phased array having a spatially periodic geometry must be avoided.
In accordance with the invention described in the above-referenced '486 Hildebrand et al application, and diagrammatically illustrated in FIGS. 2 and 3, this unwanted sidelobe/grating effect is minimized by using a spatially aperiodic phased array geometry, in which a plurality of N antenna elements (such as dipole elements) 31, 32, 33, . . . , 3N are unequally distributed or spaced apart from one another in a two-dimensional, generally planar array 30, shown as lying along a circle 40 having a center 41. This unequal distribution is effective to decorrelate angular and linear separations among elements of the array.
Each dipole 3i of the circular array is oriented orthogonal to the plane of the array, so as to produce a directivity pattern that is generally parallel to the plane of the array. Via control of amplitude and phase weighting elements coupled in the feed for each dipole element, the composite directivity pattern of the array is controllably definable to place a main lobe on a desired user, and one or more nulls along (N-1) radial lines `r` emanating from the center 41 of the array toward adjacent cells containing potential interfering co-channel users. Namely, for any angle of incidence of a received signal, the vector distance from any point along that radial direction to any two elements of the array is unequal and uniformly distributed in phase (modulo 2.pi.).
What results is a spatially decorrelated antenna element separation scheme, in which no two pairs of successively adjacent antenna elements have the same angular or chord separation. Without spacial correlation among any of the elements of the array, sidelobes of individual elements, rather than constructively reinforcing one another into unwanted composite sidelobes of substantial magnitude, are diminished, thereby allowing nulls of substantial depth to be placed upon co-channel interferers.
As further described in the '287 Halford et al application, non-limiting examples of weighting coefficient algorithms that may be employed for determining the values of the weighting coefficients and thereby the directivity pattern of the base station's phased array antenna include the "Maximum SNR Method," described in the text "Introduction to Adaptive Arrays," by R. Monzingo et al, published 1980, by Wiley and Sons, N.Y., and the PSF algorithm described in U.S. Pat. No. 4,255,791 (the '791 patent) to P. Martin, entitled: "Signal Processing System," issued Mar. 10, 1981, assigned to the assignee of the present application and the disclosure of which is herein incorporated.